Polyurethane foams are materials made by reaction of a polyol and an organic polyisocyanate in the presence of a catalyst, a blowing agent, and a foam-stabilizing surfactant; often, other additives are included as well.
A wide variety of each of these reagents is known, and foams having vastly differing properties may be produced by appropriate selection of particular combinations of ingredients. Thus, for example, foam formulations based on polyester polyols, polyether polyols, and mixtures thereof are known, as are formulations which produce rigid foams, conventional flexible foams, and high resilience foams.
The surfactants which are used in each of the several types of foam are generally siloxane-polyether block copolymers which differ from one another depending upon the particular types of compositions in which they are to be employed. Such siloxane polyether copolymers are of two classes-"hydrolyzable" siloxane-polyether block copolymers in which the polyether blocks are attached to the siloxane blocks via Si--O--C linkages, and "nonhydrolyzable" siloxane-polyether block copolymers in which the polyether blocks are attached to the siloxane blocks via Si--C linkages.
Siloxane-polyether block copolymer surfactants are very complex mixtures of structures, the precise compositional nature of which are not readily definable. For a given nominal "nonhydrolyzable" siloxane-polyether copolymer the siloxane block has relatively broad molecular weight distribution. Within the siloxane block molecular weight distribution there are sub-distributions based on the number and spacing of the attachment sites for the polyether blocks. The polyethers which are attached to the siloxane are also complex distributions of various molecular species, and frequently are blends of polyethers of different molecular weights and polarities.
The platinum catalyst used to promote hydrosilylation, a reaction which is commonly employed to prepare such silicone-polyether copolymers by addition of an Si--H moiety across the double bond of an allyl-started polyether, is also poorly defined. Some evidence suggests this catalyst is homogeneous and some suggests that it is heterogeneous. Trace impurities in the silicone and allyl-containing polyether reactants have a dramatic impact on the hydrosilylation reaction. When the reactants are relatively free from platinum inhibitors the hydrosilylation is rapid and the order in which the polyethers react appears to be substantially random. The presence of certain trace impurities appears to affect both the reaction rate and the selectivity. There are also side reactions such as dehydrocondensation, which causes the loss of polyether attachment sites and crosslinks the siloxanes.
Thus, the potential exists for considerable structural variability in the product surfactant if the proper parameters are not precisely controlled during its preparation. The extent to which the above variables affect the quality and consistency of the resulting siloxane-polyether copolymer surfactants is very dependent upon the nominal structure of the particular surfactant being produced, and the way in which the reactants are prepared and purified.
For each surfactant of a given nominal structure and made by a given process there is a practical optimum "ceiling performance" level. Most of the lots of the surfactant will perform in a given urethane foam at or near this level, but some preparations of the material, ostensibly identical, will perform somewhat less well. For best performance and consistency of the surfactant, it would be very desirable to have the means to modify surfactants to improve their performance. Prior to the present invention, however, such means for improving the performance of siloxane-polyether copolymer surfactants intended for use in the manufacture of conventional flexible polyurethane foam apparently have not been disclosed in the technical or patent literature, and do not appear to be known to the art.
It has been known for a number of years that organic sulfonate or carboxylate salts can be used in combination with certain silicone surfactants for rigid polyurethane foam and polyester foam. In some cases these organics were used as the sole surfactant. The ability of organic sulfonate or carboxylate salts to function as co-surfactants or as the exclusive surfactant in these two classes of foam has been commonly thought to be due to the high polarity of polyols used for rigid and polyester foams (see U.S. Pat. No. 3,594,334 for discussion on surface tension lowering).
Anionic organic surfactants do not function as surfactants in conventional flexible polyurethane foam. It is postulated that this is because they are not sufficiently surface active to adequately reduce the surface tension of the less polar polyethers used for conventional flexible polyurethane foam.
U.S. Pat. No. 4,751,251 discloses a surfactant composition for rigid polyurethane and rigid polyisocyanurate foams, this surfactant composition including from about 15 to about 50 percent each of (1) an organic surfactant, (2) a siloxane-polyoxyalkylene surfactant, and (3) water, alcohol of 1 to 3 carbon atoms, or a mixture thereof.
U.S. Pat. No. 4,686,240 deals with a process for producing conventional flexible foams and high resilience polyurethane foams using foam modifiers which are alkali metal or alkaline earth metal salts of Bronsted acids having pK.sub.a values greater than one. Among the several classes of foam modifiers disclosed are salts of alkali and alkaline earth metals and carboxylic acids. These foam modifiers are used with conventional silicone-polyether surfactants in foam preparations. The anions of the salts can be attached to a polyol, a polyether, a silicone-polyether copolymer, or a hydrocarbon. Generally, the level of the foam modifier employed is a substantial fraction of the sum of the foam modifier and the surfactant. In the foam preparation process, the required ingredients are mixed and immediately foamed. The '240 patent excludes aryl sulfonate salts as foam modifiers, because the pKa of the parent sulfonic acid is less than 1, which is the minimum value permitted under this patent. There is no indication that the foam modifiers are used in conjunction with the surfactants.
U.S. Pat. No. 4,184,022 discloses rigid polyisocyanurate foam based on a carbocyclic ester polyol and a resole polyether polyol, and employing a catalyst which includes an alkali metal salt of a carboxylic acid. The foam-forming composition may also include surfactants. Carboxylate salts such as these are commonly used as catalysts in isocyanurate foams.
U.S. Pat. No. 4,146,498 deals with additives for improving certain properties of polyester urethane foam. The foam-forming compositions contain an organic surfactant and may also include a silicone surfactant. The specification indicates that both the silicone surfactant and the organic surfactant are employed in amounts up to about 5 parts per hundred of the polyol reactant.
U.S. Pat. No. 4,107,069 deals with precursors for rigid polyurethane foams, these precursors containing both alkali metal salts of carboxylic acids and silicone surfactants.
U.S. Pat. No. 4,088,615 discloses the use of organic surfactant and silicone surfactant in polyester urethane foams. These materials are employed in approximately the same amounts.
U.S. Pat. No. 3,821,122 discloses that the cloud points (i.e., the water solubility) of hydrolyzable silicone surfactants for conventional flexible polyurethane foam may be improved by forming an alkali salt of a sulfonic acid having 10 to 35 carbon atoms in situ in the surfactant during its preparation.
U.S. Pat. No. 3,669,913 discloses the use of a nonionic organic surfactant in combination with a nonhydrolyzable siloxane oxyalkylene surfactant in conventional flexible polyurethane foam.
U.S. Pat. No. 3,594,334 discloses the use of certain combinations of anionic organic surfactants (sulfonates and carboxylates) with siloxane-polyoxyalkylene copolymer surfactants for stabilization of flexible polyester urethane foams.
U.S. Pat. No. 3,562,786 discloses mixtures of cationic, anionic, nonionic or amphoteric organic surfactants with siloxane-oxyalkylene copolymer surfactants, the organic surfactants being employed to reduce the surface tension of aqueous solutions of surfactants used in applications such as wetting and foaming. The siloxane-oxyalkylene copolymer surfactants of the present invention are neither suggested by the extremely broad and general disclosure of the reference, nor shown by any of the examples, which illustrate surfactants having from 0 to a maximum of 20 dimethyl siloxy units in the siloxane. Use of the compositions of the reference in flexible polyurethane foams was apparently not contemplated.
German Patent 2615804, abstracted in Chemical Abstracts Vol. 88, 90441p, discloses mixtures of a siloxane-polyether and sulfonated castor oil used as stabilizers for polyurethane foams based on mixtures of polyether polyols and polyester polyols. The sulfonated castor oil was employed at a level of four times that of the siloxane-polyether surfactant.
Chemical Abstract Volume 87, No. 185851r discusses a composition containing 20% to 65% of a siloxane-polyoxyethylene block copolymer surfactant in combination with 10% to 25% of an anionic surfactant, for stabilization of ester polyurethanes.
Chemical Abstract Volume 109, No. 39033g, discloses the use of silicone surfactants in combination with organic surfactants for stabilization of polyester polyurethane foams.
The use of an amine carboxylate salt as a catalyst in polyester foam is taught in European patent application E.P. 63-930.
Although the art has recognized that organic surfactants and silicone surfactants may be used in specialized foams such as rigid foams or polyester foams, as discussed above, the presently claimed surfactant compositions for use in conventional flexible polyurethane foams have not been suggested.